ADI~NOSINE
changes in energy status -whether this reflects shortcoming,'," of the methods or a failure of the hypothesis remains to be resolved.
If cytosolic ADP and AMP concentrations are elevated dudng net A'rP catabolism, then the ADP-stimulated AMP-selective 5'-nucleofidase that has been purified from the hearts of several species may have a role in the formation of cytasolic adenosine. Indeed, strong evidence has accumulated for the existence of such an intracallular adenosine pool based on experiments involving separation of cells and medium and on establishing that there is a pool of adenosine that can be metabolized by cytosolic enzymes, including SAH hydrolase and adenosine kinase. The major unanswered question is whether this adenosine pool can suffer export via the nuclanalde transporter in sufficient quantities to act on extracallular receptors. If so then the "retaliatory' pathway is complete.
Tests of the concepts regarding adenosine metabolism have been limited by the inability to manipulate the activities of all of the individual enzymes, especially the 5'nucleotidases. The application of molecular biology techniques promises a new and more powerful approach, however.